British Prime Minister David Cameron said Assad's move to allow aid would not prevent his eventual "day of reckoning".

Speaking at an EU summit in Brussels, where it is expected that fresh sanctions will be imposed on the regime, Cameron said it was "vitally important" that humanitarian aid reached Homs.

He said: "What we are going to be discussing today though is the situation in Syria which is absolutely appalling and it is vitally important that there is humanitarian access in to Homs and elsewhere so that people can get the help they need.

"But above all, what I think matters, is building the evidence and the picture so we hold this criminal regime to account and make sure that it is held to account for the crimes it is committing against its people and that one day, no matter how long it takes, there will be a day of reckoning for this dreadful regime."